1. Field of the Invention
The present invention relates to a method for processing a sheet of material. More particularly, it relates to methods of forming blind holes in a sheet of material.
2. Description of the Related Art
A blind hole is a hollow projection with an opening at one end and closed at the opposite end. Blind holes have wide applications ranging from locking mechanisms to electronic devices. They are useful in situations where through holes would render a less refined appearance than desired. Blind holes are also useful in applications requiring air-tightness, such as in hard disk drives where airflow may contaminate the interior of the hard drive. In such applications, blind holes may be formed in the casing for mounting printed circuit boards (PCB), motors, and semiconductor chips.
A conventional method of forming a blind hole begins by drilling a through hole in sheet material. One end of the through hole is then closed by placing a seal over that end. One problem arising from this method is that the depth of the hole is limited to the thickness of the sheet material. Using a thicker sheet of material will enable the construction of a deeper blind hole. However, in portable devices, where weight and form factor are crucial having a thick sheet of material is not practical. Another problem arises with the use of seals to close the through hole. The typical seals or plugs may not provide an airtight seal, and hence the seal would not be an adequate seal for a blind hole of a hard disk drive casing.
Another method of forming a blind hole is by controlling the depth of drilling into a sheet material. Unfortunately, one of the problems arising from this method is that the depth of the hole is limited to the thickness of the sheet material. Further, drilled material bits may be trapped in the blind hole and require additional effort to remove. Removing the debris from the blind hole is usually accomplished by using compressed air, which may lodge flying debris on other parts of the sheet material.
Another conventional method of forming a blind hole begins by compressing the sheet material against a die having an orifice therein. A portion of the sheet material is then extruded into the orifice by the compression to form an extruded portion by employing a ram press. A post is then forcibly inserted into the extruded portion of the sheet material so that the extruded portion forms a hollow projection around the post The insertion of the post is controlled so that a through hole will not be formed.
When the post is withdrawn, a blind hole or hollow projection having a closed end at the sheet material is formed. One problem with this compression method is that a recess or indentation may be formed at the side of the compressed sheet when the sheet material is forcibly extruded into the orifice. This deformation may be a thinned and weakened spot in the closed end of the blind hole. Another problem is that the sheet material experiences stretching and stress from the compression, which may weaken the sheet material or even cause failure.
Additionally, with this method, the sidewall height of the blind hole projecting or extruding from the sheet material can be increased, but at the expense of the sidewall thickness of the blind hole sidewall. To achieve an increased extrusion height, a post having a larger diameter is required. Accordingly, the sidewall is necessarily thinned out to provide material for the increased height. If the blind hole is further tapped to cut screw threads, the sidewall will be weakened and prone to breakage.
In view of the foregoing, it is therefore desirable to have a method and apparatus for forming blind holes in a sheet material hating increased projection or extrusion height and greater sidewall thickness.